pidgin/pidgin

Convert the gplugin subproject to use the auto fallback stuff of dependency.

Testing Done:
Verified the the purples gir's loaded and were able to call into gplugin under python, when built as a subproject and against a system installed version.

Reviewed at https://reviews.imfreedom.org/r/937/
xmppdisco_SOURCES = [
'gtkdisco.c',
'gtkdisco.h',
'xmppdisco.c',
'xmppdisco.h'
]
xmppdisco_resource = gnome.compile_resources('xmppdiscoresources',
'resources/xmppdisco.gresource.xml',
source_dir : 'resources',
c_name : 'xmppdisco')
xmppdisco_SOURCES += xmppdisco_resource
xmppdisco = library('xmppdisco', xmppdisco_SOURCES,
dependencies : [libpurple_dep, libpidgin_dep, glib],
name_prefix : '',
install : true, install_dir : PIDGIN_PLUGINDIR)